The former world No1 said she had taken meldonium, a heart medicine which improves blood flow, for a decade following various health problems including regular sicknesses, early signs of diabetes and “irregular” results from echocardiography exams. “I was first given the substance back in 2006. I had several health issues going on at the time,” she said. Sharapova did not specify whether she had used it constantly since then.

Meldonium was banned because it aids oxygen uptake and endurance and several athletes in various international sports have already been caught using it since it was banned.

The Latvian company Grindeks, which manufactures meldonium, told the Associated Press that four to six weeks is a common course. “Depending on the patient’s health condition, treatment course of meldonium preparations may vary from four to six weeks. Treatment courses can be repeated twice or thrice a year,” the company said in an emailed statement.

“Only physicians can follow and evaluate patient’s health condition and state whether the patient should use meldonium for a longer period of time.”

While Grindeks has previously stated that the drug can provide an “improvement of work capacity of healthy people at physical and mental overloads and during rehabilitation period,” the company said that it believes the substance would not enhance athletes’ performance in competition and might even do the opposite.

“It would be reasonable to recommend them to use meldonium as a cell protector to avoid heart failure or muscle damage in case of unwanted overload,” the company said. Grindeks said that, in sports activity, the drug slows down how the body breaks down fatty acids to produce energy.

Grindeks did not comment when asked whether someone with the symptoms Sharapova described would be a suitable patient for meldonium. The company said it was designed for patients with chronic heart and circulation conditions, those recovering from illness or injury and people suffering with “reduced working capacity, physical and psycho-emotional overload”.

The AP was able to buy vials and tablets of meldonium over the counter in Moscow on Tuesday. Accompanying documentation stated that side-effects could include blood pressure changes, irregular heartbeat and skin conditions.

A German anti-doping expert Mario Thevis, who helped to develop the test for meldonium, said that testing was reliable despite the drug’s recent addition to the Wada banned list.

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“As a scientist, you can never say 100%, but the substance is non-natural, which means that it can be readily differentiated [from naturally occurring substances],” said Thevis, a professor at the anti-doping laboratory in Cologne. “It can be tested as reliably as any other doping agent.”

He added: “There is a potential of the substance to enhance performance and it has been described as a means to facilitate recovery and to enhance physical as well as mental workload capabilities.”

Thevis said the drug appeared most prevalent in Eastern European countries. “The fact that the substance is available as a prescription drug as well as over the counter in certain formulations, and it is only approved, such a substance, in Eastern European, Baltic countries, the probability is that the athlete who is originating from that region has more contact to that sort of substance. It’s a logical consequence,” he said.

Although meldonium was banned in January, the decision to ban it had been announced by Wada and sports organisations as early as September 2015. Sharapova said she received an email with information on the changes in December but did not read it at the time.